Organic composites are built from polymeric matrix and solid, hard particulate or fibrous reinforcement. Typical reinforcing materials are inorganic fillers such as silica, talc, alumina, glass spheres, calcium carbonate, ceramic powders, silicon carbide, inorganic fibers such as glass, carbon, ceramic, boron and organic fibers such as kevlar, cellulose, lignin, and nylon. When the particles of the added solid material are small enough and are compatible with the polymeric matrix, the properties of the mixture are nonlinear, due to the interaction polymer-particle on the molecular level (Peter L. Maul, Nanocor Incorporated Corporate Technical Center, Arlington Heights, Ill. USA, in “Plastic nanocomposites: the concept goes commercial. Such composite materials are termed nanocomposites and exhibit better strength and order (permeability, orientation etc.).
In order to stabilize the composition of polymer matrix and additives some mediating agent is necessary. Surfactants are known to stabilize solutions composed of immiscible solvents. The same phenomenon occurs in polymers where polymers of different molecular structure upon mixing together by melting or in solution, tend to separate into multi-phase structure resulting in a mixture having inferior physical properties compared to the original polymer components. In order to mix together mixtures of different polymers (having different basic repeating units, molecular weight, branching rate, polymers which differ in their end and pendant groups or in the nature of stereoisomerism, polymers with a different degree of crosslinking or of acid-base interactions), surfactants-like entities should be added to the polymeric mixture. These surfactant-like entities known as compatibilizers, stabilize the polymeric blend and give rise to improved mechanical, physical and chemical properties of the blend. The added compatibilizers which are polymeric, stabilize the phases and enable creating stable, homogeneous multi-phase compositions, with good stress transfer between phases, with practical value (Datta Sudhin, Loshe David J. Polymeric compatibilizers—uses and benefits in polymer blends., Hanser Publishers 1996). Compatibilizers, in addition to stabilizing polymer-polymer interactions, further serve at polymer-filler interface (Eastman publication APG-10, July 1998), and especially in polymer-cellulose interface (Andrzej M. Krzysik and others, “Wood-polymer bonding in extruded and nonwoven web composite panels”). In the case where a hydrophilic filler or reinforcement like cellulose is mixed together with a hydrophobic matrix (e.g. polyethylene or polypropylene) the presence of the compatibilizer is crucial. In such a case, the compatibilizer blocks the hydroxyl groups and seals the surface of the particle (U.S. Pat. No. 6,117,545; M. Krishnan & R. Narayan “Compatibilization of biomass fibers with hydrophobic materials” Mat. Res. Soc. Symp. Proc. (1992) 266, 93-104) The disadvantage of compatibilizers limiting their use is their relatively high price and high viscosity. Furthermore, the high viscosity dictates that they be mixed only in high-shear/high-temperature equipment (extruder for example). The process of incorporating it into the complicated mixture is energy and time consuming and the targeting of the compatibilizer to a specific surface is limited. Also their formulation is very sensitive to processing conditions, and their treatment is limited to the outer surface of particles and fibers, a severe drawback when dealing with porous particles. One more drawback that it is very difficult to target these additives to specific surface and thus high percentage of the additive is consumed on irrelevant surfaces/fillers pores.
Combinations of compatibilizer and nano or microscopic porous fillers are problematic due to the high surface area of the filler.
Another approach to stabilize a composition of polymer(s) and additives may be the use of coupling agents. These agents, unlike compatibilizers that encapsulate the particle/polymer phase, are low molecular weight reactive molecules that have multifunctionality that enable the chemical bridging between solid and polymer (Tailoring Surfaces with Silanes”, Chemtech, Vol. 7, 766-778, 1977). The mode of action of the coupling agents is by forming covalent/ionic bonds to the different components. Their advantages are: good penetration into porous materials, high reactivity, inorganic compatibility, ease of application utilizing relatively low cost mixing equipment. However, they are volatile (imparing economic and environmental problems), and tend to migrate from interfaces—thus being poor compatibilizers. In addition, their chemical reactivity spectrum is rather limited.
Pretreatment of cellulosic fillers by low molecular weight reactive monomers and olygomers are described in M. Krishnan & R. Narayan “Compatibilization of biomass fibers with hydrophobic materials” Mat. Res. Soc. Symp. Proc. (1992) 266, 93-104, and in Rajeev Karnani et al., “Biofiber-Reinforced Polypropylene Composites” Polymer Eng. & Sci. (1997) 37, 476-483. The prior art use simple but relatively expensive ingredients like isocyanates or silanes. The mechanical properties of the result interface are brittle and the design flexibility in properties is limited. Cellulosic fiber composites and nanocomposites are described for example in U.S. Pat. No. 6,103,790—“Cellulosic microfibril reinforced polymers and their application”, U.S. Pat. No. 5,973,035—“Cellulosic fiber composites”, and U.S. Pat. No. 6,066,680—“Extrudable composite of polymer and wood flour”.